University of Birmingham
Kinetics of stabilised Criegee intermediates derived
from alkene ozonolysis : reactions with SO2, H2O
and decomposition under boundary layer
conditions
Newland, Mike; Rickard, Andrew R.; Alam, Mohammed; Vereecken, Luc; Muñoz, Amalia;
Ródenas, Milagros; Bloss, William
DOI:
10.1039/C4CP04186K
License:
Creative Commons: Attribution (CC BY)
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Publisher's PDF, also known as Version of record
Citation for published version (Harvard):
Newland, MJ, Rickard, AR, Alam, MS, Vereecken, L, Muñoz, A, Ródenas, M & Bloss, WJ 2015, 'Kinetics of
stabilised Criegee intermediates derived from alkene ozonolysis : reactions with SO2, H2O and decomposition
under boundary layer conditions' Physical Chemistry Chemical Physics, vol 17, no. 6, pp. 4076-4088. DOI:
10.1039/C4CP04186K
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Cite this: Phys. Chem. Chem. Phys.,
2015, 17, 4076
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Kinetics of stabilised Criegee intermediates
derived from alkene ozonolysis: reactions with
SO2, H2O and decomposition under boundary
layer conditions
Mike J. Newland,a Andrew R. Rickard,b Mohammed S. Alam,a Luc Vereecken,c
Amalia Muñoz,d Milagros Ródenasd and William J. Bloss*a
The removal of SO2 in the presence of alkene–ozone systems has been studied for ethene, cis-but-2-ene,
trans-but-2-ene and 2,3-dimethyl-but-2-ene, as a function of humidity, under atmospheric boundary layer
conditions. The SO2 removal displays a clear dependence on relative humidity for all four alkene–ozone
systems confirming a significant reaction for stabilised Criegee intermediates (SCI) with H2O. The observed
SO2 removal kinetics are consistent with relative rate constants, k(SCI + H2O)/k(SCI + SO2), of 3.3 (1.1) 105 for CH2OO, 26 (10) 105 for CH3CHOO derived from cis-but-2-ene, 33 (10) 105
for CH3CHOO derived from trans-but-2-ene, and 8.7 (2.5) 105 for (CH3)2COO derived from
2,3-dimethyl-but-2-ene. The relative rate constants for k(SCI decomposition)/k(SCI + SO2) are 2.3 (3.5) 1011 cm3 for CH2OO, 13 (43) 1011 cm3 for CH3CHOO derived from cis-but-2-ene, 14 (31) 1011 cm3 for CH3CHOO derived from trans-but-2-ene and 63 (14) 1011 cm3 for (CH3)2COO.
Uncertainties are 2s and represent combined systematic and precision components. These values are
derived following the approximation that a single SCI is present for each system; a more comprehensive
interpretation, explicitly considering the differing reactivity for syn- and anti-SCI conformers, is also presented.
Received 17th September 2014,
Accepted 23rd December 2014
This yields values of 3.5 (3.1) 104 for k(SCI + H2O)/k(SCI + SO2) of anti-CH3CHOO and 1.2 (1.1) 1013
for k(SCI decomposition)/k(SCI + SO2) of syn-CH3CHOO. The reaction of the water dimer with CH2OO is
DOI: 10.1039/c4cp04186k
also considered, with a derived value for k(CH2OO + (H2O)2)/k(CH2OO + SO2) of 1.4 (1.8) 102.
The observed SO2 removal rate constants, which technically represent upper limits, are consistent with
www.rsc.org/pccp
decomposition being a significant, structure dependent, sink in the atmosphere for syn-SCI.
1. Introduction
Atmospheric oxidation processes are central to understanding trace
gas atmospheric composition, the abundance of air pollutants
harmful to human health, crops and ecosystems, and the removal
of reactive greenhouse gases such as methane. The principal
atmospheric oxidants responsible for initiating the gas-phase
degradation of volatile organic compounds (VOCs), NOx and SO2
are OH, NO3 and O3, with additional contributions from other
species such as halogen atoms. Recent field measurements1 in
a
University of Birmingham, School of Geography, Earth and Environmental
Sciences, Birmingham, UK. E-mail: [email protected]
b
Wolfson Atmospheric Chemistry Laboratories, Department of Chemistry,
University of York, York, UK and National Centre for Atmospheric Science,
University of York, York, UK
c
Max Planck Institute for Chemistry, Atmospheric Sciences, J.-J.-Becher-Weg 27,
Mainz, Belgium
d
Instituto Universitario CEAM-UMH, EUPHORE Laboratories,
Avda/Charles R. Darwin. Parque Tecnologico, Valencia, Spain
4076 | Phys. Chem. Chem. Phys., 2015, 17, 4076--4088
a boreal forest have identified the presence of an additional
oxidant species, removing SO2 and producing H2SO4. The
additional SO2 oxidation observed was substantial (comparable
to that due to OH radicals alone), and attributed to a product of
alkene ozonolysis – the boreal forest environment being one in
which substantial biogenic alkene emissions occur – suggested
to be the stabilised Criegee intermediate (SCI). The gas-phase
oxidation of SO2 in the atmosphere is of interest to climate
research as it leads to formation of H2SO4, contributing to new
particle formation and sulphate aerosol loading, in competition
with condensed phase oxidation.2 A missing mechanism for the
conversion of SO2 to H2SO4 could lead to model underestimation
of the sulphate aerosol burden and affect radiative forcing
calculations,3 with corresponding implications for climate predictions. Enhanced SO2 oxidation in alkene–ozone systems was
first reported by Cox & Penkett4 over forty years ago, however
the precise reaction mechanism giving rise to this effect remains
elusive. The ‘‘Criegee’’ ozonolysis reaction mechanism was first
postulated in the 1940s.5 It is now accepted that the ozone
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Scheme 1 Simplified mechanism for the reaction of Criegee Intermediates
(SCIs) formed from alkene ozonolysis.
molecule adds to the CQC double bond via a concerted cycloaddition to form a primary ozonide, followed by cleavage of the C–C
bond and one of the O–O bonds forming a carbonyl molecule and a
carbonyl oxide, or ‘Criegee intermediate’ (CI).6 Ozonolysis derived
SCIs are formed with a broad internal energy distribution, to yield
chemically activated and stabilised SCIs. SCIs can have sufficiently
long lifetimes to undergo bimolecular reactions with H2O and SO2
amongst other species. Chemically activated SCIs may also undergo
collisional stabilisation, unimolecular decomposition or isomerisation (Scheme 1). For substituted alkenes, SCIs can undergo a 1,4-Hshift rearrangement through a vinyl-hydroperoxide (VHP) via the
so-called ‘‘hydroperoxide channel’’ and decompose to yield OH and
a vinoxy radical – a substantial non-photolytic source of atmospheric oxidants.7,8 This is the favoured channel for SCIs formed
in the syn-configuration. Time resolved studies9 show that the
VHP may persist for appreciable timescales under boundary layer
conditions, giving rise to the observed pressure dependence of OH
radical yields,10 and opening the possibility for bimolecular
reactions of this species to occur. SCIs formed in the anticonfiguration are thought to primarily undergo rearrangement
and decomposition via a dioxirane intermediate (‘‘the acid/
ester channel’’), producing a range of daughter products and
contributing to the observed overall HOx radical yield.6,11
k1
Alkene þ O3 ! fSCI þ ð1 fÞCI þ RCHO
k2
SCI þ SO2 ! SO3 þ RCHO
k3
SCI þ H2 O ! products
kd
SCI ! products
(R1)
(R2)
(R3)
(R4)
Until recently, it has been thought that the predominant
atmospheric fate for SCIs was reaction with water vapour12,13 –
leading to a significant source of organic acids and hydroperoxides,
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suggesting that bimolecular reaction with SCIs is an unimportant
oxidation mechanism for trace gas species. This view was recently
challenged by direct observation14 and kinetic studies15–17 of
the CH2OO and CH3CHOO SCIs.
Taatjes and co-workers,15 directly observing CI kinetics for
the first time, found reactions of CH2OO with SO2 and NO2 to
be substantially faster than previously thought, pointing to a
potentially important role for this species in atmospheric SO2
and NO2 oxidation; subsequent measurements have identified
SO3 (ref. 16) and NO3 (ref. 18) as products of these reactions.
The key to whether SCIs are indeed significant contributors to
gas-phase atmospheric SO2 oxidation is the ratio of the rate
constants for reaction of the SCI with SO2 (k2), to that with H2O
(k3) and decomposition (kd). In laboratory studies where SCIs
were produced by the 248 nm laser photolysis of alkyl iodide
precursors at 4 Torr total pressure, with the SCI decay monitored
by VUV photoionisation in the presence of excess co-reactants,
this ratio has recently been reported to be 103–104 for the smallest
two SCIs (CH2OO15,17 and CH3CHOO16), with k2 on the order of
1011 cm3 s1 and k3 on the order of 1015 cm3 s1. In contrast,
alternative studies of SO2 oxidation in alkene–ozone systems,
performed at atmospheric pressure through detection of the
H2SO4 product, find much smaller SCI + SO2 rate coefficients
(by ca. two orders of magnitude).19 Explanations for this apparent
discrepancy may include the lifetime of the secondary ozonide
(SOZ) adduct formed from the SCI + SO2 reaction, collisionally
stabilised at atmospheric pressure,20 effects of the presence of
multiple SCI conformers with differing reactivity, or contributions
from other oxidant species, formed within the ozonolysis system,
to SO2 reaction. Understanding the behaviour of the ozone–
alkene–SO2 system in the presence of water vapour is critical
to quantifying the impact of SCI chemistry upon atmospheric
SO2 oxidation.21
An additional, potentially important, fate of SCI under atmospheric conditions is unimolecular decomposition (denoted
kd in (R4)). For CH2OO, rearrangement via a ‘hot’ acid species
represents the lowest accessible decomposition channel, but
due to lack of alkyl substituents, the theoretically predicted
298 K rate constant is rather low, 0.3 s1.22 Previous studies
have identified the hydroperoxide rearrangement as dominant
for SCIs with a syn configuration, determining their overall
unimolecular decomposition rate.7,8 For syn-CH3CHOO recent
experimental23 work yielded a decomposition rate of 3–30 s1.
Theoretical work24 has predicted a decomposition rate coefficient
of 24.2 s1 for syn-CH3CHOO and 67.2 s1 for anti-CH3CHOO
(for which only the ester channel exists), owing to the potential
energy release from the higher energy anti-conformer.23 An upper
limit to total CH3CHOO loss (decomposition and heterogeneous
wall losses combined) of o250 s1 has been reported by Taatjes
et al.16 Earlier experimental work reported decomposition rate
constants of r20 s1 for CH3CHOO derived from cis-but-2-ene
ozonolysis,25 and 76 s1 (accurate to within a factor of three)
for CH3CHOO derived from trans-but-2-ene ozonolysis.26 For
(CH3)2COO (derived from 2,3-dimethyl-but-2-ene ozonolysis) a
total loss rate of 3.0 s1 has recently been determined experimentally.19 This value (which represents an upper limit for kd)
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is somewhat smaller than but comparable in magnitude to an
earlier measurement of 6.4 s1 (determined at 100 Torr).27 Theoretical estimates of (CH3)2COO decomposition rates are higher, at up
to 250 s1.22 Photolysis loss rates have also recently been reported
for CH2OO28 and CH3CHOO29 of 1 s1 and 0.2 s1 respectively,
calculated for actinic flux values at midday, SZA = 01.
Here, we present results of a series of experiments in which
the oxidation of SO2 during the ozonolysis of ethene, cis-but-2-ene,
trans-but-2-ene and 2,3-dimethyl-but-2-ene (tetramethylethylene,
TME), was investigated in the presence of varying amounts of
water in the European Photochemical Reactor facility (EUPHORE),
Valencia, Spain.
2. Experimental
2.1
EUPHORE
EUPHORE is a 200 m3 simulation chamber used for studying
reaction mechanisms under atmospheric boundary layer conditions. In general, experiments comprised time-resolved measurement of the removal of SO2 in the presence of an alkene–ozone
system, as a function of humidity. SO2 and O3 abundance were
measured using conventional fluorescence and UV absorption
monitors, respectively; alkene abundance was determined via
FTIR spectroscopy. The precision of the SO2 and O3 monitors
were 0.25 and 0.47 ppbv respectively (evaluated as 2 standard
deviations of the measured value prior to SO2 or O3 addition).
The chamber is fitted with large horizontal and vertical fans
to ensure rapid mixing (three minutes). Further details of the
chamber setup and instrumentation are available elsewhere.30,31
Experiments were performed in the dark (i.e. with the chamber
housing closed; j(NO2) r 106 s1), at atmospheric pressure
(ca. 1000 mbar) and temperatures between 296 and 303 K, on
timescales of ca. 20–30 minutes. Chamber dilution was monitored
via the first order decay of an aliquot of SF6, added prior to each
experiment. Cyclohexane (ca. 75 ppmv) was added at the beginning
of each experiment to act as an OH scavenger, such that SO2
reaction with OH was calculated to be r1% of the total chemical
SO2 removal in all experiments.
2.2
Experimental approach
Experimental procedure, starting with the chamber filled with
clean air, comprised addition of SF6 and cyclohexane, followed
by water vapour, O3 (ca. 500 ppbv) and SO2 (ca. 50 ppbv). A gap
of five minutes was left prior to addition of the alkene, to allow
complete mixing. The reaction was then initiated by addition of
the alkene (ca. 500 ppbv for ethene, 200 ppbv for cis- and transbut-2-ene and 400 ppbv for TME). The chamber was monitored
for an hour subsequent to the addition of ethene and forty five
minutes for cis- and trans-but-2-ene and TME. The rate of alkene–
ozone consumption is dependent on k1. Roughly 25% of the
ethene was consumed after an hour, while for cis- and trans-but-2ene and TME 90% of the alkene was consumed within roughly
25 minutes, 20 minutes and 6 minutes respectively. Each
experiment was performed at a constant humidity, which was
increased in a step-wise manner for consecutive runs to cover
4078 | Phys. Chem. Chem. Phys., 2015, 17, 4076--4088
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the range 1.5–21% RH. Measured increases in [SO2] agreed with
measured volumetric addition across the SO2 and humidity
range used in the experiments.
2.3
Analysis approach
The following sections describe (i) a common analysis applied
to all systems, but which features several approximations, and
(ii) more detailed consideration of each chemical system in turn,
to address potential contributions from the water dimer, and
multiple criegee species within each system (e.g. contrasting
reactivity of different SCI conformers).
From the chemistry presented in reactions (R1)–(R4) it is
assumed that SCI will be produced in the chamber from the
reaction of the alkene with ozone at a given yield, f. The SCI
produced can then react with SO2, with H2O, with other species
or decompose under the experimental conditions applied. The
rate at which SO2 is lost, compared with the total production of
SCI, is determined by the fraction, f, of the total SCI produced
which reacts with SO2, compared to the sum of the total loss
processes of the SCI (eqn (E1)):
f ¼
k2 ½SO2 k2 ½SO2 þ k3 ½H2 O þ kd þ L
dSO2
¼ff
dO3
(E1)
(E2)
Here, L is the sum of any other pseudo-first order chemical loss
processes for SCI in the chamber, after correction for dilution. (E2)
neglects other (non-alkene) chemical sinks for O3, such as reaction
with HO2 – also produced directly during alkene ozonolysis,11 but
indicated through model calculations to account for o2% of ozone
loss under all the experimental conditions of this work. Eqn (E1)
and (E2) treat the SCIs formed as a single species – this is the
case for (e.g.) ethene and TME, but is an approximation for the
2-butenes; considered further below.
2.3.1 SCI yield calculation. Values for fSCI were determined
for each ozonolysis reaction from experiments performed under
dry conditions (RH o 1%) in the presence of excess SO2
(ca. 1000 ppbv), such that SO2 scavenges the overwhelming
majority of the SCI. From eqn (E2), regressing dSO2 against
dO3 (corrected for chamber dilution), assuming f to be unity
(i.e. all the SCI produced reacts with SO2) determines the value of
fmin, a lower limit to the SCI yield. Fig. 2 shows the experimental
data, from which fmin was derived, for all four alkene ozonolysis
systems studied.
The lower limit criterion applies as in reality f will be less
than one, at experimentally accessible SO2 levels, as a small
fraction of the SCI will still react with any H2O present, or
undergo decomposition. The actual yield, f, was determined by
combining the results from the high-SO2 experiments with those
from the series of experiments performed at lower SO2, as a
function of [H2O], to determine k3/k2 and kd/k2 (see Section 2.3.2),
through an iterative process to determine the single unique value of
fSCI which fits both datasets. It is important to note that the SCI
yield is to an extent an operationally defined quantity – for example,
OH formation from alkene ozonolysis is known to proceed over
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at least hundreds of milliseconds9 following the alkene–ozone
reaction, and so the corresponding CI population must also be
evolving with time. In this work, SCI yields reflect the amount of
SCI available to oxidise SO2 on timescales of seconds to minutes.
2.3.2 k(SCI + H2O)/k(SCI + SO2) and kd/k(SCI + SO2). To
determine k3/k2 and kd/k2, a series of experiments were performed for each alkene, in which the SO2 loss was monitored as
a function of [H2O]. From eqn (E2), regression of the loss of
ozone (dO3) against the loss of SO2 (dSO2) (both corrected for
dilution, measured through the removal SF6, added at the start
of each experiment and monitored via FTIR) for an experiment
determines the factor ff at a given point in time. This quantity
will vary through the experiment as SO2 is consumed, and
other potential SCI co-reactants are produced, as predicted by
eqn (E1). A smoothed fit was applied to the experimental data
for the cumulative consumption of SO2 and O3, DSO2 and DO3,
(Fig. 1) to determine dSO2/dO3 (and hence ff) at the start of
each experiment, for use in eqn (E3). The start of the experiment (i.e. when [SO2] B 50 ppbv) was used as this corresponds
to the greatest rate of production of the SCI, and hence largest
experimental signals (O3 and SO2 rate of change) and is the point
at which the SCI + SO2 reaction has the greatest magnitude
compared with any other potential chemical loss processes for
either species (see discussion below).
1
k3
kd þ L
1 ¼ ½H2 O þ
½SO2 f
k2
k2
the case for the ethene and TME systems, for the but-2-ene
systems this is an approximation as two conformers of the
CH3CHOO SCI (syn and anti) are produced. Further analysis
is performed in Section 3.3.2 in which the SO2 loss in the but-2ene systems is treated as having two components, related to the
different SCI.
3. Results and discussion
(E3)
The value [SO2]((1/f) 1) can then be regressed against [H2O]
for each experiment to give a plot with a gradient of k3/k2 and an
intercept of (kd + L)/k2 (eqn (E3)). Our data cannot determine
absolute rate constants (i.e. values of k2, k3, kd) in isolation, but is
limited to assessing their relative values, which may be placed on
an absolute basis through use of an (external) reference value.
Eqn (E1)–(E3) as presented above assume that only a single
SCI species is present in each ozonolysis system. While this is
Fig. 1 Cumulative consumption of SO2 and O3, DSO2 versus DO3, for
the ozonolysis of four alkenes in the presence of SO2 at a range of
relative humidities from 1.5–21%. Open symbols are experimental data,
corrected for chamber dilution. Solid lines are smoothed fits to the
experimental data.
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Fig. 2 DSO2 vs. DO3 during the excess SO2 experiments, to determine the
minimum SCI yield for the four alkenes.
3.1
Introduction
Table 1 shows the resulting SCI yields obtained for the ozonolysis of
ethene, cis-but-2-ene, trans-but-2-ene and tetramethylethylene (TME);
uncertainties are 2s, and represent the combined systematic
(estimated measurement uncertainty) and precision components.
Also shown are past literature values, obtained under various
conditions using a range of different SCI scavengers.
The yield of CH2OO from ethene ozonolysis obtained in this
work is 0.37 (0.04). This yield has been investigated in many
previous studies, with values ranging from 0.34–0.50 determined – a more detailed review is available elsewhere.31 The
yield obtained in this work is at the lower end of but within the
envelope of these estimates, and is in excellent agreement with
the current IUPAC recommendation of 0.37.
The yield of CH3CHOO from cis-but-2-ene ozonolysis
obtained in this work is 0.38 (0.05), with that from transbut-2-ene being 0.28 (0.03). These values fall within the range
of reported literature values of 0.18–0.43 and 0.13–0.53 for cisbut-2-ene and trans-but-2-ene respectively (Table 1). cis and
trans-but-2-ene both yield (differing) mixtures of syn and anti
conformers of CH3CHOO, the relative amounts of which are not
well known, and which are treated here initially as a single SCI
species (this approximation is discussed further in Section 3.3).
Berndt et al.32 recently reported a yield of 0.49 (0.22) for the
CH3CHOO produced from trans-but-2-ene ozonolysis (also
treating both syn and anti conformers as a single SCI species).
The yield of (CH3)2COO from TME ozonolysis obtained in
this work is 0.32 (0.02). This again falls within the (wide)
range in the literature of 0.10–0.65, with Berndt et al.32 most
recently reporting a yield of 0.45 (0.20).
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Table 1 SCI yields derived in this work and reported in the literature.
Uncertainty ranges (2s, parentheses) indicate combined precision and
systematic measurement error components for this work, and are given
as stated for literature studies. All referenced studies were conducted
between 700 and 760 Torr. C2B – derived from cis-but-2-ene; T2B – derived
from trans-but-2-ene
SCI
jSCI
CH2OO
0.37
0.37
0.35
0.39
0.47
0.50
0.39
0.54
Ref.
(0.04)
(0.05)
(0.053)
(0.05)
(0.04)
(0.11)
(0.15)
This work
MCMv3.2a (IUPAC)33
Niki et al.34
Hatakeyama et al.35
Horie and Moortgat36
Neeb et al.37
Hasson et al.38
Alam et al.31
CH3CHOO
(C2B)
0.38 (0.05)
0.19
0.18
0.43
This work
Rickard et al.39
Niki et al.40
Cox and Penkett41
Fig. 3 Application of eqn (E3) to derive rate constants for reaction of CH2OO,
CH3CHOO (derived from cis-but-2-ene; C2B), CH3CHOO (derived from transbut-2-ene; T2B), and (CH3)2COO with H2O (k3) and decomposition (kd), relative
to that for reaction with SO2, k3/k2 and (kd + L)/k2 – see text.
CH3CHOO
(T2B)
0.28
0.49
0.53
0.45
0.19
0.42
0.24
0.13
This work
Berndt et al.32
Berndt et al.19
Cox and Penkett41
Hatakeyama et al.35
Horie and Moortgat36
Hasson et al.38
Rickard et al.39
Table 2 SCI relative rate constants derived in this work using the singleSCI water monomer approach (i.e. eqn (E3); see text). Uncertainty ranges
(2s, parentheses) indicate combined precision and systematic measurement error components
(CH3)2COO
a
(0.03)
(0.22)
(0.24)
(0.03)
(0.07)
0.32 (0.02)
0.45 (0.20)
0.62 (0.28)
ca. 0.65b
0.10 (0.03)
0.30
0.11
http://mcm.leeds.ac.uk/MCM/.
of 0.15 at 0 Torr).
b
This work
Berndt et al.32
Berndt et al.19
Drozd et al.10
Hasson et al.38
Niki et al.40
Rickard et al.39
At 700–760 Torr (low pressure limit
Fig. 1 shows the cumulative consumption of SO2 relative to
that of O3, DSO2 versus DO3 (after correction for dilution), as a
function of [H2O] for each experiment for the four alkenes
studied. A fit to each experiment, extrapolating the experimental
data to evaluate dSO2/dO3 at t = 0 (start of each experimental run)
for use in eqn (E1)–(E3), is also shown. The overall change in
SO2, DSO2, is seen to decrease substantially with increasing
humidity (over a relatively narrow range of RH (1.5–20%)) for
all four alkenes. This trend would be expected from the understood chemistry ((R1)–(R4)), as there is competition between SO2,
H2O, and decomposition for reaction with the SCI.
Fig. 3 shows a fit of eqn (E3) to the data for each alkene,
giving a slope of k3/k2, and an intercept of (kd + L)/k2. The results
appear to show a generally linear relationship; however, for cisand trans-but-2-ene and TME, the data point at the highest relative
humidity accessible in this work ([H2O] = 1.5–2.0 1017 cm3)
appears to deviate from this relationship. These data points lie
outside the 95% confidence intervals defined by all the other
(lower relative humidity) data for each alkene. For the analysis to
determine k3/k2 and (kd + L)/k2 presented in Table 2, the points at
the highest RH are excluded and the kinetic parameters are
derived from a linear fit to the measurements from all other
experiments. Extended analyses to account for the non-linearity
4080 | Phys. Chem. Chem. Phys., 2015, 17, 4076--4088
SCI
105 k3/k2
1011 cm3 kd/k2
CH2OO
CH3CHOO (C2B)
CH3CHOO (T2B)
(CH3)2COO
3.3 (1.1)
26 (10)
33 (10)
8.7 (2.5)
2.3 (3.5)
13 (43)
14 (31)
63 (14)
observed for CH3CHOO and (CH3)2COO are presented in the
following sections.
One potential explanation for the observed curvature in the
CH3CHOO and (CH3)2COO data is measurement error. DSO2 is
relatively small at high [H2O] compared to the precision of the
measurements; however, even allowing for associated uncertainties,
the points at high RH do not fit the linear relationship successfully
applied to the remaining data points. Moreover, any systematic error
in the measurement of O3, SO2 or H2O would also be expected to
affect the results for the ethene system (and to a greater extent, given
the slow ethene–ozone reaction rate and consequent lower overall
chemical SO2 loss observed), suggesting that the cause lies in
contrasting chemical behaviour. In terms of experimental factors,
H2O was measured using multiple approaches (two dew-point
hygrometers in addition to a solid state probe) with no evidence
for any divergence with RH. SO2 monitors can exhibit humiditydependent interferences (quenching of the SO2 signal), commonly of
the order of a few percent, observed at very high RH, and corrected
through incorporation of a nafion dryer (fitted in this case); in
addition the monitor-derived SO2 concentration increments were in
agreement with those calculated from the measured SO2 addition
and chamber volume, across the relative humidity range studied.
It should be noted that the kd values reported here represent
upper limits, as a consequence of possible further chemical losses
for the SCI within our experimental system (as represented by
L in eqn (E3), notwithstanding the approach of extrapolating to
the start of each experiment to minimise these). Other potential
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fates for SCIs include reaction with ozone,42,43 other SCI,43
carbonyl products,44 acids,45 or with the parent alkene43 itself.
Sensitivity analyses indicate that the reaction with ozone could
be significant, as predicted by theory42,43 with a possible contribution of up to 10% of SCI loss for (CH3)2COO at 2% RH, while total
losses from reaction with SCI (self-reaction), carbonyls and alkenes
are calculated to account for o1% of the total SCI loss under the
experimental conditions applied.
3.2
CH2OO
3.2.1 Linear Fit. For CH2OO a linear fit (i.e. using eqn (E3)
vs. H2O) describes the observations well (Fig. 3) over the entire
[H2O] range studied (1–20 1016 molecules per cm3). This fit
gives relative rates k3/k2 of 3.3 (1.1) 105 and (kd + L)/k2 of
2.3 (3.5) 1011 cm3 (Table 2).
These relative rates can be placed on an absolute basis using
absolute measurements of k2(SCI + SO2). In Table 3 we apply
the absolute k2 values reported by Welz et al.,15 obtained using
direct methods at reduced pressure (4 Torr), to the relative rates
shown in Table 2. Using this method, the value obtained for
k3(CH2OO + H2O) is 1.3 (0.4) 1015 cm3 s1. This is
consistent with the recent determination by Welz et al.,15 that
k3 o 4 1015 cm3 s1, but is (ca. 14–50 times) greater than the
recent estimates of Ouyang et al.18 (k3 = 2.5 (1) 1017 cm3 s1)
and Stone et al.17 (k3 o 9 1017 cm3 s1).
The derived (kd + L) value for CH2OO using this method is
8.8 (13) s1, i.e. zero within uncertainty. Theoretical work22
has predicted kd(CH2OO) to be small (B0.3 s1), in agreement
with the experimentally derived value reported here.
3.2.2 CH2OO + (H2O)2. Recent experimental work46 has
reported the reaction of CH2OO with the water dimer, (H2O)2,
(reaction (R5)) to be very fast (1.1 1011 cm3 s1 – assuming k2 =
3.9 1011 cm3 s1 (ref. 15)), in broad agreement with theoretical
predictions,47 but in contrast to other experimental work.45
k5
CH2 OO þ ðH2 OÞ2 ! products
(R5)
The CH2OO data from this study appear to be well described
by a linear fit under the experimental conditions applied
(a fast reaction of CH2OO with (H2O)2 would be manifested
as a significant upward curvature in Fig. 3). However, this does
not mean the results are inconsistent with reaction of CH2OO
with (H2O)2.
In Fig. 4, eqn (E4) (an expanded version of eqn (E3), including
the SCI + (H2O)2 reaction (R5)) is applied to the data, now
expressed in terms of (H2O)2, calculated for each RH via the
equilibrium constant.48
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
kd
1
k3 ðH2 OÞ2
k5 1 ¼
þ
½SO2 0
ðH2 OÞ2 þ
(E4)
f
k2
k2
k2
Kp
The value for k3/k2 (water monomer) derived from the fit shown
in Fig. 4 is 2.5 (0.7) 105. It is seen that this value is rather
insensitive to the inclusion of the (H2O)2 term in eqn (E4) as the
value is within the uncertainties of the linear fit to the data
presented in Fig. 3 – see also Table 2. Converting this value to an
absolute value using the k2 from Welz et al.15 gives k3 = 9.9 (2.9) 1016 cm3 s1. The derived value of kd/k2 is 6.4 (66) 1010 cm3, which, using the Welz et al.15 k2 gives an absolute value
for kd of 1.5 (16) s1. This is again indistinguishable from zero,
within uncertainty, as is the kd determined from eqn (E3) (Fig. 3).
Note the large uncertainties in kd, resulting from allowing three
parameters to vary in the optimisation; consequently kd was fixed to
zero in eqn (E4) to determine the k3/k2 and k5/k2 values.
The resulting value of k5/k2 (water dimer) is 1.4 (1.8) 102.
Converting this to an absolute value using the k2 from Welz
et al.15 gives k5 = 5.6 (7.0) 1013 cm3 s1. This is roughly a
factor of twenty smaller than the value derived by Berndt et al.,46
but within a factor of two of the upper limit for k5 deduced by
Welz et al. (o3 1013 cm3 s1) (ref. 45) from the data presented
by Stone et al.17 The inset plot in Fig. 4 also shows two additional
fits generated using eqn (E4) with k3/k2 fixed to 9.9 1016 and
kd fixed to zero. One fit line uses the k5 value reported by Berndt
et al.46 (blue dashed line). This is seen to overestimate the
presented data. The green dotted line shows a fit to the upper
limits of the uncertainties of the measured data. This yields
a k5/k2 value of 0.10 (0.01), giving an upper limit k5 value of
3.9 (0.39) 1012 cm3 s1.
Table 3 Comparison of SCI relative rate constants derived in this work to relative and absolute values from the literature. Uncertainty ranges (2s)
indicate combined precision and systematic measurement error components
SCI
105 k3/k2
1015 k3 (cm3 s1)
1011 kd/k2 (cm3)
kd (s1)
Ref.
Method
Conditionsa
CH2OO
3.3 (1.1)
1.3 (0.4)b
o4
o0.09
2.3 (3.5)
8.8 (13)b
This work
Welz et al.15
Stone et al.17
Ethene ozonolysis
Alkyl iodide photolysis
Alkyl iodide photolysis
298–303 K
4 Torr; 298 K
200 Torr; 295 K
CH3CHOO
26 (10)
33 (10)
12 (4.5)c
15 (4.5)c
anti 10 (4); syn o4
13 (43)
14 (31)
59 (196)c
64 (141)c
This work
This work
Taatjes et al.16
Berndt et al.32
Fenske et al.26
C2B ozonolysis
T2B ozonolysis
Alkyl iodide photolysis
T2B ozonolysis
T2B ozonolysis
296–302 K
297–302 K
4 Torr; 298 K
293 K
296 K
This work
Berndt et al.32
TME ozonolysis
TME ozonolysis
298–299 K
293 K
8.8 (0.4)
12 (0.1)
76 (25–228)
(CH3)2COO
8.7 (2.5)
o0.4
d
2.1 (0.6)
63 (14)
42 (3)
151 (35)d
a
Experiments were conducted at atmospheric pressure unless stated otherwise. b Assuming k2 = 3.9 1011 cm3 s1 (Welz et al.15). c Assuming
k2 = 4.55 1011 cm3 s1. Average of k2 for syn and anti CH3CHOO conformers from Taatjes et al.16 d Assuming k2 = 2.4 1011 cm3 s1. k2 for synCH3CHOO from Taatjes et al.16
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Fig. 4 Application of eqn (E4) to derive rate constants for reaction of
CH2OO with H2O (k3/k2) and (H2O)2 (k5/k2) relative to that of CH2OO with
SO2. Inset: eqn (E4) as shown in the main figure (red line), (E4) applied using
the dimer reaction rate (k5) reported by Berndt et al.45 (1.1 1011 cm3 s1)
(dashed line) and a fit of (E4) to the upper limits of the uncertainties in the
ethene data (solid green line).
The contribution of (H2O)2 to the removal of CH2OO increases
in relation to that of H2O as [H2O] increases. Hence at typical
atmospheric [H2O] (B2.5–7.5 1017 molecules per cm3), greater
than was accessible in this study, reaction with (H2O)2 could
become the dominant sink for CH2OO. In this case using
just the H2O monomer kinetics in models would considerably
underestimate the total effect of water on removal of CH2OO in
the atmosphere.
3.3
CH3CHOO
3.3.1 Single SCI Approach. The CH3CHOO data shown in
Fig. 3 for both cis and trans-but-2-ene appear to be well
described (within the uncertainties) by a linear fit to eqn (E3),
with the exception of the experiment at the highest RH ([H2O] =
1.8–1.9 1017 cm3) in both cases. The kinetic parameters derived
from a linear fit to the data (Fig. 3), using eqn (E3) (which treats the
system as producing a single SCI), excluding the highest RH
experiments, are shown in Table 2. Very similar results are obtained
for k3/k2 for CH3CHOO derived from both cis- and trans-but-2-ene
ozonolysis, with values of 26 (10) 105 and 33 (10) 105
respectively. The (kd + L)/k2 obtained for CH3CHOO from cis-but-2ene ozonolysis is 13 (43) 1011 molecule cm3 and from transbut-2-ene ozonolysis 14 (31) 1011 molecule cm3. Berndt
et al.32 reported the k3/k2 ratio from trans-but-2-ene ozonolysis
to be 8.8 (0.4) 105 (also assuming a single SCI system),
a factor of 3.75 smaller than that reported here.
The relative rate constants (Table 2) can be placed on an
absolute basis using the measurements of k2(SCI + SO2) reported
by Taatjes et al.16 (derived using the same methodology as
for CH2OO) (Table 3). As eqn (E3) treats the SCI produced as a
single SCI, we use an average of the syn and anti conformer
rates presented in Taatjes et al.,16 4.55 1011 cm3 s1. Using
this method, the value obtained for k3(CH3CHOO + H2O) from
cis-but-2-ene ozonolysis is 12 (4.5) 1015 cm3 s1 and from
trans-but-2-ene ozonolysis is 15 (4.5) 1015 cm3 s1. Taking
a mean of the k3 values reported for the two CH3CHOO conformers
4082 | Phys. Chem. Chem. Phys., 2015, 17, 4076--4088
Paper
by Taatjes et al.16 gives a value of 7.0 1015 cm3 s1, while
Sheps et al.49 give a mean value of 12 1015 cm3 s1. The
values obtained for (kd + L) are 59 (196) s1 from cis-but-2-ene
and 64 (141) s1 from trans-but-2-ene. Clearly there is a large
uncertainty associated with the kd determined from this analysis.
Fenske et al.26 have reported kd(CH3CHOO) from trans-but-2-ene
ozonolysis to be 76 s1 (accurate to within a factor of three).
3.2.2 Two conformer system. In Fig. 1 it is evident that
dSO2/dO3 falls rapidly with increasing [H2O] for all but-2-ene
experiments as RH is initially increased, but that the experiments at higher RH all appear to display a similar dSO2/dO3,
i.e. the trend in decreasing SO2 removal with increasing H2O
levels off. From this observation it appears that there may be
competing H2O dependencies to the SO2 loss present. This is
manifested in Fig. 3 as a curving over of the data at high RH.
We propose two possible explanations for this observation:
firstly that it arises from differing kinetics of the two CH3CHOO
conformers formed in but-2-ene ozonolysis; secondly that the
behaviour may reflect the presence of an additional oxidant
being formed in the ozonolysis system that reacts with SO2 but is
less sensitive to H2O. The first of these possibilities is discussed
below, the second is discussed in relation to (CH3)2COO in the
following section.
One explanation for the observed non-linearity at high RH
apparent in Fig. 3 is the differing reactivities of the syn- and
anti-conformers of CH3CHOO produced in the ozonolysis of
cis- and trans-but-2-ene. It has been predicted50 that the anticonformer reacts with H2O several orders of magnitude faster
than the syn-conformer, while the rate constant for the SCI
reaction with SO2 has been determined experimentally16 to be
about a factor of three greater for the anti-conformer than the
syn-conformer. The fraction of each conformer that is lost
to reaction with SO2 can be considered in the same way as
illustrated in eqn (E2), leading to eqn (E5) and (E6) below, plus
simplifications outlined in the following text. The total loss of
SO2 to CH3CHOO is then the sum of the fractional loss to each
conformer, multiplied by the relative SCI yield (g) (i.e. jsyn/j) of
that conformer (eqn (E7)).
f syn ¼
f anti ¼
½SO2 ½SO2 ksyn
ksyn
ksyn
3
d
½SO2 þ syn ½H2 O þ syn ½SO2 þ dsyn
k2
k2
k2
½SO2 ½SO2 anti k
kanti
kanti
½SO2 þ 3anti ½H2 O þ danti ½SO2 þ 3anti ½H2 O
k2
k2
k2
f = gsynf syn + gantif anti
(E5)
(E6)
(E7)
Eqn (E8) can then be fitted to the data presented in Fig. 3 for
cis- and trans-but-2-ene (Fig. 5).
1
1
1 ¼ ½SO2 syn syn
½SO2 1
(E8)
f
g f þ ganti f anti
Here we make two assumptions to reduce the degrees of
freedom and hence make the problem tractable with the
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dataset available. First it is assumed that ksyn
3 [H2O] may be
neglected, in keeping with theoretical predictions24 predicting
syn
ksyn
d to be over three orders of magnitude greater than k3 [H2O].
50
Further theoretical work predicts a rate constant for synCH3CHOO + H2O of 2.39 1018 cm3 s1, and recent experi16
mental work49 yields an upper limit for ksyn
cm3 s1.
3 of o2 10
syn
Hence with kd expected to be relatively fast, given the decomposition rate presented here for (CH3)2COO (a syn-conformer)
and the facile decomposition route available via the hydroperoxide mechanism for the syn-conformer, it can be assumed that
syn
ksyn
d c k3 [H2O] at the experimental conditions reported here.
Second it is assumed that kanti
{ kanti
d
3 [H2O] under the experimental conditions used. If the kinetics derived from treating
the but-2-ene data in Fig. 3 as representing a single SCI are
dominated by the anti-conformer then the kd derived from
these kinetics, which is indistinguishable from zero within
the uncertainties, suggests that kanti
is small. Taatjes et al.16
d
anti
14
report k3 to be 1.0 (0.4) 10
cm3 s1, while Sheps et al.49
report a value of 2.4 (0.4) 1014 cm3 s1. Thus, even at the
lowest [H2O] considered here (B1 1016 cm3), loss of antiCH3CHOO to H2O would be 4100 s1, and decomposition
negligible in comparison (including a kanti
of 50 s1 changes the
d
anti
syn
derived k3 and kd values by o5%).
Fitting eqn (E8) to the data shown in Fig. 5 derives a range of
anti
values for kanti
and ksyn
and gsyn
3
d dependent on the values of g
used. As the CIs, once formed and thermalised, are expected to
show the same kinetic behaviour in these experiments irrespective
Fig. 5 Fits of eqn (E8) to the cis-but-2-ene and trans-but-2-ene data
shown in Fig. 3. gsyn and ganti are 0.45 and 0.55 for cis-but-2-ene and 0.25
and 0.75 for trans-but-2-ene (see Fig. 6).
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anti
syn
Fig. 6 The ranges of kanti
and ksyn
determined from fitting
3 /k2
d /k2
eqn (E8) to the but-2-ene data (Fig. 5). The colour legend shows the
fraction of the total CH3CHOO formed that is syn-CH3CHOO (gsyn).
of their precursor alkene, the measurements from the cis-but-2-ene
and trans-but-2-ene experiments can be used in combination to
syn
syn
constrain kanti
and ganti from each alkene. Fig. 6
3 , kd and also g
anti
syn
plots the k3 vs. kd determined at different gsyn and ganti from
cis-but-2-ene and trans-but-2-ene. Where these two lines intercept represents the unique solution for both kanti
and ksyn
3
d and for
syn
anti
g and g
(Table 4).
anti
Fig. 6 determines kanti
to be 3.5 (3.1) 104 and
3 /k2
syn syn
13
kd /k2 to be 1.2 (1.1) 10 cm3. The 2s uncertainties
presented are, unsurprisingly, large as there are two free parameters. Using the relevant values of k2 for the syn and antiCH3CHOO conformers from Taatjes et al.16 to place the relative
rate constants on an absolute basis gives a value for kanti
of 2.3
3
1
anti
(2.1) 1014 cm3 s1 and for ksyn
of
288
(275)
s
.
This
k
is
d
3
comparable to (a factor of two greater than) that reported by
Taatjes et al.16 (1.0 (0.4) 1014). Novelli et al.23 have recently
1
reported ksyn
d to be an order of magnitude smaller (3–30 s ) based
on direct observation of OH formation during trans-but-2-ene
ozonolysis at atmospheric pressure.
The point of interception in Fig. 6 also determines the
relative yields of the two conformers, gsyn and ganti (which in
turn has been used to derive the optimised fits shown in Fig. 5).
For cis-but-2-ene these are determined as 0.45 and 0.55 for gsyn
and ganti respectively. For trans-but-2-ene they are determined
as 0.25 for gsyn and 0.75 for ganti. The analysis performed in this
section has implications for the determination of the SCI yield.
Using the relative rate constant k3/k2(anti-CH3CHOO) obtained,
as shown in Table 4, it is calculated that B90% of the antiCH3CHOO produced in the SCI yield experiments would react
with SO2. From the determined kd/k2(syn-CH3CHOO) it is
calculated that B67% of the syn-CH3CHOO produced in the
SCI yield experiments would react with SO2. Applying these
(with the corresponding syn and anti yields shown in Table 4)
corrections to jmin determines total SCI yields of 0.29 for transbut-2-ene and 0.42 for cis-but-2-ene. These values both lie within
the uncertainties in the SCI yields presented in Table 1 for the
two but-2-ene systems.
It is not practicable to assess the possible contribution of
the water dimer to the SCI loss for CH3CHOO because of the
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Table 4 Kinetic parameters derived for syn-CH3CHOO and anti-CH3CHOO from two-component fits to the trans-but-2-ene (T2B) and cis-but-2-ene
(C2B) experiments (see Fig. 5). Also relative (g) and absolute (j) yields of the two conformers
ga
4
14
CH3CHOO
10 k3/k2
10
syn
—
—
o0.4
o0.02
3
1
k3 (cm s )
13
10
3
kd/k2 (cm )
1.2 (1.1)
j
1
kd (s )
T2B
C2B
T2B
C2B
Ref.
Conditionsb
288 (275)
0.25
0.45
0.07
0.17
This work
Taatjes et al.16
Sheps et al.49
Novelli et al.23
—
4 Torr
200 Torr
735 Torr
0.75
0.55
0.21
0.21
This work
Taatjes et al.16
Sheps et al.49
—
4 Torr
200 Torr
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20 (3–30)
anti
a
3.5 (3.1)
gsyn = jsyn/j.
b
2.3 (2.1)
1.0 (0.4)
2.4 (0.4)
—
—
Experiments were conducted at atmospheric pressure unless stated otherwise and temperatures from 293 to 303 K.
number of free parameters that would result for a small dataset.
However, theoretical predictions47 suggest that this may be less
important for CH3CHOO than for CH2OO, indicating k(H2O)2/
k(H2O) to be two orders of magnitude smaller for anti-CH3CHOO
than for CH2OO.
3.4
(CH3)2COO
3.4.1 Linear fit. For (CH3)2COO a value of 8.7 (2.5) 105
(Table 2) was obtained from a linear fit to the data in Fig. 3,
excluding the experiment performed at the highest RH ([H2O] =
1.6 1017 cm3) from the regression. (kd + L)/k2 for (CH3)2COO was
determined as 63 (14) 1011 molecule per cm3. These values
can be placed on an absolute basis using the measurements of
k2(syn-CH3CHOO + SO2) reported by Taatjes et al.16 as there are no
reported k2 values for (CH3)2CHOO from direct experiments and
(CH3)2COO is a syn-conformer (i.e. there is always a methyl
group in a syn orientation to the terminal oxygen). This gives
values for k3 ((CH3)2COO + H2O) of 2.1 (0.6) 1015 cm3 s1
and for kd of 151 (35) s1 (Table 3).
Berndt et al.32 have recently reported the k3/k2 ratio for
(CH3)2COO to be o0.4 105 (i.e. approximately a factor of 22
lower than the relative rate reported in this study). Theoretical
predictions50 also suggest k3 to be very slow, 3.9 1017 cm3 s1.
No measured values have been reported for kd ((CH3)2COO), but
a more facile overall decomposition than for CH2OO or the mean
of the CH3CHOO isomers might be anticipated as the vinylhydroperoxide isomerisation channel26 is always available.
3.4.2 Additional oxidant. In the case of the SCI formed from
TME ozonolysis, (CH3)2COO, there is always a methyl group in a syn
position to the carbonyl oxide moiety, thus the analysis presented in
Section 3.3.2 for the CH3CHOO isomers does not apply. A possible
alternative explanation for the observed behaviour (and possible
contributor to the behaviour observed in the but-2-ene systems)
is that there is a further oxidant (X) of SO2, in addition to the SCI,
being formed during the ozonolysis reaction. If this oxidant reacts
relatively slowly with H2O, it could give rise to the apparent ‘two
component’ nature of the observations seen in Fig. 3. It may also
provide an alternative explanation of the observed nature of the SO2
loss from the but-2-ene experiments or could be occurring in
addition to the effects of differing conformer reactivities.
Eqn (E9) (below) is an expanded version of (E2), in which we
consider the contribution from a second SO2 oxidant, making
4084 | Phys. Chem. Chem. Phys., 2015, 17, 4076--4088
the approximation that this species does not react appreciably
with water vapour. In eqn (E9), f is the sum of f SCI (the fraction
of SCI reacting with SO2) and f x, each multiplied by the relative
yield of the total oxidant (i.e. SCI + X) gSCI and gx. Following the
assumption of negligible H2O reactivity, (dSO2/dO3)x in eqn (E9)
can be derived from the SO2 loss at the highest RH experiments
(i.e. when all the SO2 loss is attributed to X + SO2) of B10 ppbv.
Therefore, loss of SO2 from reaction with X, relative to the loss
of O3 (dSO2/dO3)x is approximately 0.025. j represents the total
oxidant yield (i.e. jSCI + jx). Assuming that gx is not dominant
(o0.5), then j, as calculated from correcting jmin as in Section 3.1,
changes little (0.31–0.34) from the value presented in Table 1.
Eqn (E10) is then an expanded version of eqn (E3) that includes
the additional oxidant term.
1 dSO2
SCI SCI
x x
SCI SCI
f ¼g f
þg f ¼g f
þ
(E9)
f dO3 x
1
1
½SO2 f
20
11 3
6B
7
gSCI ½SO2 1 dSO2 C
B
C 17
þ
¼ ½SO2 6
4@
5
k3
kd f dO3 x A
½SO2 þ ½H2 O þ
k2
k2
(E10)
Fig. 7 shows eqn (E10) fitted to the TME data from Fig. 3.
It is not possible to determine unique values for the parameters
included in eqn (E10) due to the degrees of freedom vs. the
limited data set. The fit shown in Fig. 7 uses values of j = 0.34,
gSCI = 0.88, k3/k2 = 6.7 104 and kd/k2 = 1.2 1012 cm3.
However, Fig. 7 does demonstrate that a two-oxidant system, as
represented by eqn (E10), is able to describe the data within
uncertainty.
Fig. 7 also includes a linear fit (i.e. eqn (E3)) to the full (CH3)2COO
dataset (including the highest RH experiment). While it seems
unlikely that the curvature observed in the data is a result of
measurement error (as described in Section 3.1), this must be
considered as a possibility for (CH3)2COO in light of the two
conformer explanation not being applicable. The linear fit in
Fig. 7 gives a k3/k2 value of 3.8 (3.2) 105 cm3 s1, a factor of
two smaller than the k3/k2 value presented in Table 2.
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Fig. 7 Data for TME ozonolysis (as shown in Fig. 3), with fit to eqn (E10),
and linear fit to the full dataset.
The most obvious candidate for an additional oxidant present to consume SO2 is OH. OH radicals are produced in the
chamber, primarily (in the absence of sunlight and NOx)
through the alkene + ozone reaction.11 However cyclohexane
was added in excess at the beginning of each experiment to act
as an OH scavenger, such that SO2 reaction with OH was
calculated to be r1% of the total chemical SO2 removal in
all experiments. Other potential candidates for this oxidant
species include the (stabilised) vinyl hydroperoxide (VHP) intermediate, a secondary ozonide (formed through an SO2–SCI
cyclic adduct), and dioxirane (Scheme 1).
Drozd et al.10 have presented evidence for substantial VHP
stabilisation (derived from the (CH3)2COO CI) at pressures of a
few hundred Torr, with a lifetime of the order of a few hundred
milliseconds with respect to decomposition to form OH, providing scope for bimolecular reactions of this species to occur.
This would be consistent with the kinetic observations presented here: a small yield in the systems with syn-SCIs, while for
ethene, no VHP intermediate is available, and the standard
chemistry ((R1)–(R4)) can be used to satisfactorily reproduce the
observations. However, no significant SO2 reactivity is known for
the peroxide or alkene functionalities present in the closed shell
VHP in isolation, hence it may be surprising if this species reacted
rapidly with SO2, although theoretical studies have suggested that
the VHP may react with H2O.23
Secondary ozonide species, formed as an adduct from the
SCI + SO2 reaction, have been suggested to account for the observed
isotopic exchange in alkene–ozone–SO2 systems,25 and are suggested
to have lifetimes of seconds or longer.19 Such a secondary ozonide
could react with a further SO2 molecule (i.e. a two-component
secondary ozonide catalysed oxidation route for SO2 to SO3 conversion), however a substantial humidity dependence to the overall
process may still be anticipated (on the basis of SCI removal through
the SCI + H2O reaction), which is not observed here.
The ‘hot acid/ester channel’ (rearrangement and decomposition via a dioxirane intermediate) is the dominant isomerisation route available for anti-SCIs. Although the hydroperoxide
channel is the principal isomerisation route for syn-SCIs, the
ester mechanism is also available,25 and it is likely that a small
proportion of the syn-CI will isomerise through this channel to
form a dioxirane. Dioxiranes are known to be highly reactive
and selective oxidising agents,51,52 and have a particular affinity
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for sulphur compounds.53 Additional oxidation of SO2 by the
dioxirane, over and above that arising directly from reaction
with the SCI, could then explain the observed behaviour of SO2
in the TME experiments (and contribute to the behaviour observed
for the but-2-ene systems). For the small CI CH2OO, formed in the
ethene system, it has been predicted that the dioxirane formed is
considerably less stable than the methyl substituted dioxiranes
formed from but-2-ene and TME ozonolysis, and furthermore will
decompose promptly due to chemical activation.
The possibility of non-CI products of the ozonolysis system
being responsible for some of the observed SO2 oxidation has
been suggested previously as an alternative (or additional)
explanation for the observed behaviour of SO2 in the atmosphere.1,54,55 In their laboratory study Berndt et al.19 note that
their data were not perfectly described by a model in which a
single (SCI related) SO2 oxidation process was assumed, and
commented that the SO2 oxidation in ozonolysis systems may
in fact be more complex. Taatjes et al.56 have recently suggested
that these data19 are consistent with a two-component oxidation system, either two processes removing SO2 in parallel (as
described above), or through a sequential two-step SO2 removal
mechanism, such as the secondary ozonide route outlined
above. However, this latter mechanism cannot (in isolation)
account for the observations presented here.
The presence of an additional oxidant would have implications for the role of alkene ozonolysis in oxidation of trace
gases in the atmosphere. If an oxidant is being formed that
reacts slowly with H2O then this, perhaps in addition to SCI,
may contribute to the additional (non-OH) SO2 oxidation
observed in recent field experiments.1 Further investigation of
this possibility is needed.
As for CH3CHOO, the analysis performed in this section has
implications for the determination of the SCI ((CH3)2COO)
yield. Using the value of kd derived from the linear fit to all
the data shown in Fig. 7 would indicate an SCI yield of 0.33.
Using the value of kd derived from the ‘additional oxidant fit in
Fig. 7, and taking into account that B10 ppb of the SO2 loss in
the high SO2 experiment would be attributed to reaction with the
additional oxidant rather than the SCI, determines a slightly
lower jmin of 0.23, and a corrected yield of 0.32, very similar to
that shown in Table 1. Both of these possible alternative SCI
yields lie within the uncertainties in the SCI yield from TME
ozonolysis presented in Table 1.
As for CH3CHOO, it is not possible to quantitatively consider
the contribution of the water dimer to the SCI loss given the
limited data, but theory53 predicts k(H2O)2/k(H2O) to be three
orders of magnitude smaller than that for CH2OO suggesting
that reaction with the water dimer would be unimportant for
(CH3)2COO at typical atmospheric boundary layer [H2O].
4. Atmospheric implications
The derived values for k3 reported in Table 3 correspond to loss
rates for reaction of SCI with H2O in the atmosphere of 650 s1 for
CH2OO, B3500 s1 for CH3CHOO and B1050 s1 for (CH3)2COO
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(assuming [H2O] = 5 1017 molecules per cm3, equivalent to an
RH of 65% at 298 K). Comparing this to the derived kd values it is
seen that reaction with H2O is predicted to be the main sink for SCI
in the atmosphere, but also that loss through decomposition
cannot be neglected for some SCIs – contributing on the order of
0–1% for CH3CHOO and 13% for (CH3)2COO.
An estimate of a mean steady state SCI concentration in the
background atmospheric boundary layer can then be calculated
using eqn (E11).
½SCI ¼
½Alkene½O3 k1 f
k3 ½H2 O þ kd
(E11)
Using the data given below, a steady state SCI concentration
of 1.7 103 molecules per cm3 is estimated for an ozonolysis
source (noting that other potential atmospheric sources of SCI
exist, e.g. photolysis of alkyl-iodides in the marine boundary
layer, and sinks, e.g. reaction with NO and NO2). This assumes an
ozone mixing ratio of 40 ppbv, an alkene mixing ratio of 2 ppbv,
j of 0.35, and mean reaction rate constants k1 (alkene–ozone)
of 1 1016 cm3 s1; k2 (SCI + SO2) of 3.5 1011 cm3 s1,
k3 (SCI + H2O) of 2 1015 cm3 s1, kd of 30 s1 with [H2O] of
5 1017 cm3 (RH B 65%).
However, in the case of CH3CHOO the data shown in Fig. 3,
and the discussion above, indicate contributions from multiple
species – syn- and anti-conformers with contrasting behaviour.
It is clear that the burden of CH3CHOO in the atmosphere
would be better described by considering these two fractions of
SO2 loss separately. Eqn (E12) expands eqn (E11) to treat the
two conformers separately, where [SCI] = [anti-SCI] + [syn-SCI],
making the same assumptions as for the analysis of CH3CHOO
in eqn (E5) and (E6). kanti
is estimated to be 1 1014 cm3 s1
3
(taking into account that CH2OO is considered as an anti-SCI in
this analysis and that the derived k3(CH2OO) is more than an
order of magnitude smaller than the derived k3(CH3CHOO) of
1
2.3 1014 cm3 s1), ksyn
and janti =
d is assumed to be 200 s
syn
j = 0.175. Additionally the anti-SCI + water dimer reaction is
also considered, using a value of 5.6 1013 cm3 s1 as derived
for CH2OO in this work.
!
fanti
fsyn
þ syn
½SCI ¼ ½Alkene½O3 k1 anti
2
kd
k3 ½H2 O þ kanti
5 Kp ½H2 O
(E12)
Using these values in eqn (E12) determines [anti-SCI] = 164
molecules per cm3 and [syn-SCI] = 4.4 103 molecules per cm3.
The formation of an additional oxidant during alkene ozonolysis would be expected to have a similar effect to the two
component contribution presented in eqn (E12) based on the
apparent yields from the experiments presented here. From
this analysis the atmospheric SCI burden is seen to likely be
dominated by syn-SCI since this term is at least an order of
magnitude greater than the anti-SCI term.
A typical diurnal loss rate of SO2 to OH (kOH [OH]) is 9 107 s1,15 while the SO2 loss rate due to reaction with SCI,
using the values derived from eqn (E12), would be 1.2 107 s1.
This suggests, for the conditions and assumptions given above, the
4086 | Phys. Chem. Chem. Phys., 2015, 17, 4076--4088
loss of SO2 to SCI to be about 13% of loss to OH. This analysis
neglects additional chemical sinks for SCI, which would reduce
SCI abundance but are unlikely to be competitive with the two
main SCI loss processes identified herein. SCI concentrations
are expected to vary greatly depending on the local environment,
e.g. alkene abundance may be considerably higher (and with a
different reactive mix of alkenes) in a forested environment,
compared to a rural background environment. The majority of
the SCI burden, particularly in forested regions, is likely to be
dominated by larger SCI derived from (C5) isoprene and (C10)
monoterpenes. The chemistry of these species could differ greatly
from the small SCI reported here (which we have found to be
structure specific, even for small alkene systems), especially for
tethered SCI derived from ozonolysis of internal double bonds
within (for example) some monoterpenes. It is clear that the total
SCI loss rate is dependent upon SCI identity and configuration, and
that further work is required to quantify speciated SCI in the
atmosphere, and to accurately calculate SCI concentrations for
use in atmospheric modelling.
5. Conclusions
It has been shown that at relatively low [H2O] (o1 1017 cm3) the
loss of SO2 in the presence of four ozone–alkene systems: ethene, cisbut-2-ene, trans-but-2-ene and 2,3-dimethyl-but-2-ene significantly
decreases with increasing water vapour. This is consistent with
production of a stabilised Criegee intermediate from the ozonolysis
reaction and subsequent reaction of this species with SO2 and H2O.
Competition between H2O and SO2 for reaction with the SCI leads
to the observed relationship which is sensitive to water vapour
abundance over a relatively narrow range of RH. Derived kinetic
data for these ozonolysis systems shows that the reaction rates are
dependent on the structure of the SCI. At [H2O] 4 1 1017 cm3 the
SO2 loss in the presence of cis- and trans-but-2-ene, and 2,3-dimethylbut-2-ene appears to show a reduced dependence upon H2O. The
results suggest that there is an H2O dependent and an H2O
independent fraction to the observed SO2 loss in these systems.
These two fractions may be attributable to differing kinetics of the
two conformers produced in but-2-ene ozonolysis or to other oxidant
products of the alkene ozonolysis reaction. This observation means
that SCI structure must be considered in atmospheric modelling of
SCI production from alkene ozonolysis, and suggests that the
atmospheric SCI burden (and hence the oxidation of trace gases)
will be dominated by syn-SCI.
This work provides constraints on the behaviour of SCI formed
through alkene ozonolysis under conditions relevant to the atmospheric boundary layer, but also highlights the complex nature
and incomplete current understanding of the ozonolysis system.
Further research is needed to definitively quantify the impact of
this chemistry upon atmospheric oxidation.
Acknowledgements
The assistance of the EUPHORE staff is gratefully acknowledged.
Marie Camredon, Stephanie La and Mat Evans are thanked for
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helpful discussions. This work was funded by EU FP7 EUROCHAMP
2 Transnational Access activity (E2-2012-05-28-0077), the UK
NERC (NE/K005448/1) and Fundacion CEAM. Fundación CEAM
is partly supported by Generalitat Valenciana, and the projects
GRACCIE (Consolider-Ingenio 2010) and FEEDBACKS (Prometeo –
Generalitat Valenciana). EUPHORE instrumentation is partly
funded by the Spanish Ministry of Science and Innovation,
through INNPLANTA project: PCT-440000-2010-003. LV is supported by the Max Planck Graduate Center with the Johannes
Gutenberg-Universität Mainz (MPGC).
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